1. Field of the Invention
The present invention relates to a moving object extension controller.
2. Description of the Related Art
A known extension mechanism for a moving object; such as, a lens of a camera has been disclosed in Japanese Patent Laid-Open No. 1-158420. This mechanism is shown in the longitudinal cross-sectional diagram of FIG. 10, wherein a cam ring 206 is rotated to advance or withdraw first and second lens frames 201 and 202 along the optical axis O.
The above mechanism will be described in detail. First, an output of a drive motor 209 is transmitted to a gear 206b formed on the outer circumference of the cam ring 206 via a gear array 208. On the other hand, the first lens frame 201 holding front lens elements 203 is united with the second lens frame 202 holding back lens elements 204. A rectilinear guide rod 210, which is locked in a fixture 213 of a barrel, is fitted in a guide hole 211 bored on the flange of the first lens frame 201 and second lens frame 202. The rectilinear guide rod 210 prevents the rotation of the first lens frame 201 and second lens frame 202 around the optical axis. A spring 212 is compressed and inserted between the first lens frame 201 and the fixture 213, which presses the first lens frame 201 and second lens frame 202 in the optical-axis direction. Thereby, a drive pin 205, which is implanted in the cylindrical outer circumference of the second lens frame 202, is continuously maintained in contact with an end cam 206a formed on the end surface of the cam ring 206 as shown in the development of FIG. 11.
The actuation of the lens drive mechanism having the foregoing construction will be described. First, the drive motor 209 is activated. Then, the cam ring 206 is driven via the gear 206b to rotate with the optical axis as the axis of rotation. The rotation causes, as shown in the development of FIG. 11, the end cam 206a and drive pin 205 to move to positions 206a' and 205' respectively along the optical axis. With the actions of the end cam 206a and spring 212, the first lens frame 201 and second frame lens 202 move in both the .alpha. and .beta. directions along the optical axis O.
Thus, when the drive motor 209 starts up, the first lens frame 201 and second lens frame 202 move along the optical axis. The quantities of movements of the first lens frame 201 and second lens frame 202 are controlled with pulses sent from a pulse output unit (not shown) that is interlocked with the cam ring 206 or gear array 208.
The aforesaid conventional lens extension mechanism employs a cam ring, while an extension mechanism shown in FIG. 12 employs a rod type lead screw, a ring type helicoid screw, or other lead screw. In FIG. 12, an output of a drive motor 109 is transmitted as a torque to a lead screw 5 via a gear array 6. Then, driven via a female screw 1a engaging with the lead screw 5, lens frames 1 and 2 whose rotations around the optical axis O are restricted i.e., are not permitted to advance or withdraw along the optical axis. The quantities of movement of the lens frames 1 and 2 are controlled with pulses sent from a photo-interrupter (PI) or other pulse output unit, which is not shown, coupled to the lead screw or gear array.
The aforesaid lens extension mechanism described in Japanese Patent Laid-Open No. 1-158420 rotates a lens having a high moment of inertia to control the extension of a lens. This deteriorates the precision in stopping the lens. A drive motor is requested to provide a high torque, which results in adoption of a large-scale drive motor. Consequently, a lens drive mechanism becomes large.
The aforesaid conventional lens extension unit using a rod type lead screw has solved the foregoing problems of the lens extension unit disclosed in Japanese Patent Laid-Open No. 1-158420. However, since a lead screw is adopted, a lead error caused by the lead screw poses a problem. The lead error is attributable to a difference .DELTA.x in the quantity of extension relative to the quantity of rotation .theta. of the lead screw between a set value TH and an actual value RE caused by a lead screw actually molded or machined.
The above error is, needless to say, present in the aforesaid ring type lead screw.
In recent years, a lens has been more powerful with its size reduced progressively. A minor difference in the quantity of extension influences resolution considerably. From this viewpoint, the aforesaid error becomes a significant problem.
The influence will be described in conjunction with FIG. 13 or the graph of the quantity of extension and FIG. 14 showing how big a circle of confusion caused by a lens is. As shown in FIG. 13, a difference in the quantity of extension relative to the quantity of rotation .theta. between a set value and an actual value is .DELTA.x, and the longitudinal magnification of a lens is .beta.2. Under these conditions, a product of .DELTA.x by .beta.2 is regarded as a deviation from a focal point of a subject at infinity, fc. Assuming that the diameter of a circle of confusion resulting from the deviation is d, the following equation is established: EQU d=K.multidot..DELTA.x.multidot..beta.2
where, K is a constant of proportion.
If the diameter of a circle of confusion, d, ranges from 30 to 50 .mu.mm, the resultant photograph appears to be in focus. If the diameter is larger than 50 .mu.mm, the resultant photograph looks out of focus. Even when .DELTA.x is constant, since the longitudinal magnification .beta.2 is high due to the recent trend toward reduction of the size of a lens, the diameter of a circle of confusion, d, cannot help but increase.
Therefore, despite the merits of improved precision in stopping a lens and of reduced size of a lens, the lens extension mechanism using a lead screw cannot be employed for a lens that is very powerful or has a high longitudinal magnification .beta.2.